In order to effectively harness the output of an internal combustion engine, motor vehicles are typically provided with a gear transmission having a plurality of speed ratios. In use, these speed ratios are selected one by one in a progressive manner, and allow a vehicle to be provided with a desirable balance of acceleration, cruising speed and fuel economy. Typically a driven axle of a heavy vehicle also provides a fixed speed ratio whereby the input (propeller shaft) speed is reduced via a pinion and crown wheel, so that the output (side shaft) speed is lower; this further ratio reduction may be in the range 3:1 to 5:1.
In some cases, the fixed ratio speed reduction provided in the rear axle is insufficient, particularly in the case of heavy vehicles that are required to move slowly in a controllable manner within a useful range of engine speed. Generally speaking it is not desirable to provide a greater speed reduction at the crown wheel and pinion, because either the pinion becomes too small to transmit the required torque, or the crown wheel is so large that ground clearance is insufficient.
In order to address this problem it has been proposed to provide hub reduction gearing at the wheel ends of an axle. Typically an epicyclic gear set is incorporated in each wheel hub, whereby the sun gear is driven by the respective side shaft, the annulus is connected to the axle (side shaft) casing, and the planet carrier is coupled to the rotatable output components, namely the wheel hub, brake drum and road wheel.
A compact hub reduction gear can be incorporated at each end of an axle, in particular where double wheels are provided, and may also allow the crown wheel and pinion ratio to be reduced, thus permitting a more robust pinion and a smaller diameter crown wheel.
The epicyclic gears of a hub reduction gear may comprise cylindrical gears. This arrangement is axially compact but is radially confined by the aperture in the center of the corresponding vehicle wheel. For this reason, and also to ensure that pinion gears have adequate strength, the range of hub reduction ratios available is approximately in the range 3:1 to 6:1.
An alternative cylindrical gear arrangement, with the annulus driven by the respective side shaft, can give a ratio range of about 1.2:1 to 1.5:1. Thus cylindrical gears do not provide a continuous spread of hub reduction ratios.
It is also possible to provide a hub reduction gear with conventional bevel gears. This arrangement is radially compact as compared with cylindrical gears, but takes up more axial space. For practical reasons of pinion size, the available range of hub reduction ratios using conventional bevel gears is about 1.5:1 to 3:1.
It will be appreciated from the foregoing that the full range of desirable speed reduction ratios cannot be provided by either cylindrical gears or bevel gears. On the other hand it is not desirable to have two different kinds of hub reduction gear with substantially different radial and axial space constraints.
Accordingly it would be advantageous to expand the ratio range of one or other of the known hub reduction gears in order to facilitate drive axles which have more components in common.